Onodi Cell-Mistaken as ICA Aneurysm

Onodi cell (sphenoethmoid air cell)

Case Submitted by Dr MGK Murthy, Mr Hari Om, Mr Venkat.

Edited by -Sumer Sethi, MD

·         Defined as ethmoidal air cell that lies posterior and sometimes superior to sphenoid sinus

·         Incidence in western world is usually 8 to 13%, though some studies give larger cadaveric incidence up to 60%

·         Optic nerve and Internal Carotid Artery  become closer than normal to  ethmoid air cells –point of concern to endoscopic sinus  surgeons

·         At places the bone separating them  is measured as mean 0.08mm. Infection may lead to mucocele

·         Occasionally could be mistaken for ICA aneurysm in view of the proximity to the loop and flow void nature on MRI. CT is usually adequate  to differentiate by presence of air density